Low Rank Coals (LRC) comprise almost 50% of total coal production in the United States, and about one-third of the coal produced worldwide. LRCs are characterized by their high levels of porosity and their water content which is retained in three basic forms: interstitial, capillary and bonded. Removal of the voids in which air, gas, and water reside in these coals requires primary comminution followed by compaction and higher energy inputs as transformation becomes more rigorous. The excess constituents, including air, gas, and water that would otherwise dilute the combustible material, are progressively expelled as interstitial voids between particles, and pores contained in the particles are eliminated.
The utility and gasification industries have long recognized the benefits of reducing these constituents in coal. Numerous beneficiation systems of varied technical complexity have been designed, but almost all use some form of thermal energy such as flue gas, steam, hot oil, hot water or the like, to remove water and some organic material (see, Davy-McKee, Inc. Comparision of Technologies for Brown Coal Drying, Coal Corporation of Victoria, Melbourne Australia (1984)). The technical, economic and environmental benefits realized by the use of these thermal drying procedures have been well documented and include increased power plant efficiency, increased generating efficiency, reduced greenhouse gas emissions, reduced dependence on carbon dioxide disposal systems, increased value of the LRC resources and reduced parasitic power consumption. But while these thermal beneficiation systems are technically effective, they are also expensive to build, costly to operate, site restricted, and must compete with other market opportunities for the energy they consume.
Additionally, thermal drying can produce coal dust that leads to unacceptably dangerous fuel products. High temperature thermal drying of coal, especially LRCs, largely alters the chemical characteristics of the fuel. The dried product is more reactive to air and may rapidly rehydrate, thus providing greater opportunity for spontaneous combustion and catastrophic fires. High volumes of coal fines and dust associated with thermally dried LRC create handling problems and product losses during rail transportation and handling, and some thermal drying systems are unable to process LRC fines of less than one-quarter inch and require alternative processing or result in substantial waste.
Thus, new coal benefication techniques are needed that can realize the substantial benefits of drying LRCs without the economic disincentives and production hazards associated with thermal drying techniques.